Adult great apes — including people — may be more playful in egalitarian societies

Chimpanzees and lowland gorillas are close relatives but their social structures are strikingly different, and Elisabetta Palagi wanted to know if these differences were reflected in the playfulness of adults.

“I am convinced that play is a perfect indicator of the degree of freedom characterizing social relationships between animals,” explains Palagi, an ethologist at the Università di Pisa in Italy.

Chimpanzees live in groups that can have several adult males, and are highly cohesive and cooperative. Lowland gorilla groups, however, are dominated by a single silverback male and social bonding is low.

In new research published in PLOS ONE, Palagi and colleagues found that adult chimpanzees are indeed more playful than adult lowland gorillas, with chimpanzees far more likely to engage in, for example, peekaboo, rope shaking, and play fighting complete with biting, kicking and slapping.

“In egalitarian species in which relationships are affected by affiliation more than hierarchy or kinship, adults frequently use play to build their relationships,” Palagi says.

Interestingly, when adult gorillas did play, it was more likely to escalate to real fighting. This was especially true when more than two gorillas joined in. This makes sense because the more players, the more inherently unstable a play session and the more difficult it is to manage.

These findings have parallels in human societies. For example, most farming communities are structured hierarchically, entailing obedience to high-ranking people as well as gender and age inequalities. “Play is generally discouraged,” Palagi says. “There is no space for creativity — it would be too risky because if a crop is lost, a whole food supply for a year is lost.”

In contrast, people in hunter-gatherer groups can be more tolerant, making decisions together, sharing food informally, and playing with each other. “The fluidity of social relationships promotes the persistence of a playful attitude,” she says.

Now, Palagi is extending her work to mechanisms of agreement in play in chimpanzees and lowland gorillas, focusing on their facial expressions and mimicry of smiles. “Play fighting is a risky affair,” she says. “If something goes wrong because I am not able to communicate my playful mood, our interaction can suddenly become aggressive.”